This invention relates to precombustion chambers in internal combustion engines, and more particularly to a cylinder head with precombustion chambers each of which is provided by inserting a hollow member of ceramics in a recess of a cast metal body of the cylinder head.
In some internal combustion engines represented by diesel engines each combustion chamber is made up of a main combustion chamber and a precombustion chamber, or simply prechamber, in which combustion is initiated. For the sake of improvements in engine startability, emission controls and/or heat insulation, studies have been made to use ceramics as the wall material for prechambers in a cylinder head. A primitive way is preparing a cast metal body of the cylinder head with generally semispherical recesses and, separately, hollow members of a ceramics good in heat insulation, heat resistance and corrosion resistance and then tightly fitting the ceramics members into the respective recesses of the cast metal body.
Actually, however, the incorporation of the hollow ceramics members encounters problems. In operation of the engine a considerable gradient of temperature is produced between the outer and inner wall faces of the ceramics member for each prechamber, so that the ceramics member is liable to break or crack by thermal shocks attributed to repeated heating and cooling. Also it is likely that the ceramics member is damaged by sharp changes in the pressure in the prechamber during each combustion stroke.
It has been tried to cope with the problem of fragility by selectively using a high-strength ceramics such as zirconia ceramics or silicon nitride ceramics. Besides, cast-insertion of the hollow ceramics members has been employed in casting of the cylinder head with a view to inducing compressive stress in the ceramic members at the stage of their insertion to thereby enhance their endurance to thermal shocks and pressure changes. However, an almost inevitable result of the cast-insertion method is degradation of dimensional precision Of the inserted ceramics members. Accordingly it is desirable to correct the induced dimensional errors by machining subsequent to the casting operation, but this is difficult when a high-strength ceramics of the aforementioned class is used since such a ceramics is quite inferior in machinability . Besides, silicon nitride ceramics are considerably high in heat conductivity and, hence, fail to provide a desired degree of heat insulation.